1. Field of the Invention
The invention relates to a rack-and-pinion mechanism and a steering system that includes the rack-and-pinion mechanism.
2. Description of the Related Art
There is an existing rack-and-pinion mechanism that converts the rotation of a pinion shaft to a reciprocating motion of a rack shaft by engaging the pinion shaft with the rack shaft. The rack-and-pinion mechanism is used in, for example, a steering system for a vehicle. The rack-and-pinion mechanism changes the steering angle of steered wheels by converting the rotation of the pinion shaft caused by a steering operation into a reciprocating motion of the rack shaft.
In this type of rack-and-pinion mechanism, the rack shaft is supported so as to be able to reciprocate in the axial direction of the rack shaft in a state in which the rack shaft is pressed to the pinion shaft by a rack guide. The rack guide is provided in a housing portion of a housing so as to be movable in a direction in which the rack guide moves toward or away from the rack shaft. The rack guide presses the rack shaft to the pinion shaft in a state in which the rack guide is pressed by an urging member provided between the rack guide and a lid member fixed to an outer opening end of the housing portion.
A clearance is formed between the rack guide and the housing portion so that the rack guide is movably housed in the housing portion. Therefore, there is a possibility that the rack guide hits the housing portion due to, for example, the movement of the rack shaft caused by a steering operation or the vibration caused when a vehicle travels. As a result, abnormal noise may be generated. Therefore, an annular fitting groove that extends in the circumferential direction of the rack guide is formed in an outer peripheral face of the rack guide, and an elastic member, for example, an O-ring is fitted in the fitting groove. In this way, an impact force generated when the rack guide hits the housing portion is reduced to suppress generation of abnormal noise (refer to, for example, Japanese Patent Application Publication No. 2005-41251). Both axial end faces of the above-described O-ring contact side faces of the fitting groove. Thus, the O-ring is fitted in the fitting groove in a state in which the axial movement of the O-ring is restricted. With this structure, it is possible to avoid the situation where the O-ring is displaced from an appropriate position when the rack guide hits the housing portion. As a result, the impact force is reliably reduced.
In order to efficiently absorb an impact force generated when the rack guide hits the housing portion with the use of the O-ring, it is preferable to increase a bulk ratio α (=S2/S1) that is a ratio of a sectional area S2 of the elastic member to a sectional area S1 of the fitting groove. However, as shown in FIG. 6, axial end faces of an O-ring 81 contact side faces 83a and 83b of a fitting groove 82, respectively. Therefore, if the bulk ratio α is increased, air is likely to be accumulated in an internal space 84, defined by the O-ring 81 and the fitting groove 82, and the pressure in the internal space 84 may be increased when the O-ring 81 is fitted in the fitting groove 82, The pressure of the air makes it difficult to fit the O-ring 81 in the fitting groove 82 in a state in which the O-ring 81 contacts a bottom face 85 of the fitting groove 82. Therefore, part of the O-ring 81 may be pushed out of a rack guide 86.
A predetermined interference is set for the O-ring so that an outer peripheral face of the O-ring is brought into close contact with an inner face of the housing portion. Therefore, the O-ring is originally fitted in the fitting groove such that the outer peripheral portion thereof projects from the fitting groove. Accordingly, if part of the O-ring is pushed out of the rack guide by the pressure of the air as described above, the O-ring is more likely to get snagged when the rack guide is housed in the housing portion, which may reduce the assembling performance. In this regard, there is still room for improvement.